Recently, as an element having an optical memory function which is necessary for realization of all-optical routers and so forth, a bistable element, above all, a semiconductor bistable element, is investigated.
Saying a word “semiconductor bistable element”, semiconductor bistable elements based on a variety of principles are reported. In particular, as a representative example in which a relatively stabilized bistable operation is realized, a semiconductor bistable element, which utilizes a structure having two different optical waveguide paths in a multi-mode interference optical waveguide, is reported by Patent Document 1 (JP-2003-084327 A) and Non-Patent Document 1 (M. Takenaka and Others, “Multimode Interference Bistable Laser Diode”, IEEE Photonics Technology Letters, Vol. 15, No. 8, pp. 1035-1037).
According to this semiconductor bistable element, although it is reported that a superior operation can be obtained as a bistable element, there are a problem (1) that a hysteresis window in a bistable operation (which is referred to as a bistable hysteresis window hereinafter) is narrow (a current must be set within a range less than several percentages of the bistable operation current), and a problem (2) that the element per se is not suitable to integration due to the fact that a full length of the element is too long.
Patent Document 2 (JP-2008-250110 A) and Non-Patent Document 2 (H. A. Bastawrous and Others, “A Novel Active MMI Bi-Stable Laser Using Cross-Gain Saturation Between Fundamental and First Order Modes”, Proceedings of The 34th European Conference on Optical Communication (ECOC 2008, Brussels, Belgium), P. 2. 15, pp 81-82, September 2008) report a bistable element in which aforesaid problems have been solved.
In particular, in Non-Patent Document 2, it has been proved that a wide operating current condition, in which a bistable hysteresis window was about 10% of the operating current, and which could not be achieved by now, can be obtained.